CATALYTIC WET AIR OXIDATION OF WASTEWATER CONTAINING

ACETIC ACID

by

TAN YANG HONG

A thesis submitted to the Faculty of Chemical and Natural Resource Engineering in

partial fulfilment of the requirement for the Bachelor Degree of Engineering in

Chemical Engineering

Faculty of Chemical and Natural Resources Engineering

Universiti Malaysia Pahang

FEBRUARY 2013

iv

TABLES OF CONTENTS

DEDICATION ii

ACKNOWLEDGEMENT iii

LIST OF TABLES vii

LIST OF FIGURES viii

LIST OF ABBREVIATIONS ix

ABSTRAK x

ABSTRACT xi

CHAPTER 1 INTRODUCTION

1.1 Background of the Study 1

1.2 Research Objectives 3

1.3 Scope of the Study 3

1.4 Significance of the Study 4

CHAPTER 2 LITERATURE REVIEW

2.1 Industrial Wastewater Containing Acetic Acid 5

2.2 Treatment Methods 6

2.2.1 Adsorption and Distillation 7

2.2.2 Membrane Processes 7

2.2.3 Wet Air Oxidation 8

2.2.4 Catalytic Wet Air Oxidation 9

2.3 Catalyst for Wet Air Oxidation 10

2.3.1 Metal Oxides and Transition Metals 10

2.3.2 Noble Metals 11

2.3.3 Potential Applications of RuO2/ZrO2-CeO2 Catalyst 11

2.3.3.1 CWAO of Phenolic Compounds 12

v

2.3.3.2 CWAO of N–Containing Compounds 12

2.4 Catalyst Synthesis Methods 14

2.4.1 Bulk Catalyst and Support Preparation 15

2.4.1.1 Precipitation Method 15

2.4.1.2 Sol-Gel Method 17

2.4.2 Impregnation Method 18

CHAPTER 3 METHODOLOGY

3.1 Research Design 21

3.2 Materials 22

3.2.1 Wastewater Containing Acetic Acid 22

3.2.2 Catalyst Reagents 22

3.2.3 Reagents of GC Analysis 23

3.3 Catalyst Preparation 23

3.4 Catalyst Testing 24

3.5 Analysis Methods 28

3.5.1 XRD and BET Analysis Method 28

3.5.2 Gas Chromatography (GC) Analysis 29

3.5.2.1 GC Specifications and Operating Conditions 29

3.5.2.2 GC Analysis Procedure 30

CHAPTER 4 RESULT AND DISCUSSION

4.1 Catalyst Characterization 31

4.1.1 X-ray Diffraction (XRD) 31

4.1.2 Physisorption Analysis (BET Method) 36

4.2 Catalyst Activity 41

4.2.1 Temperature Effect 41

4.2.2 Air Flow Rate Effect 47

vi

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 51

5.2 Recommendation 52

REFERENCES 53

APPENDICES 58

Appendix A 58

Appendix B 59

vii

LIST OF TABLES

PAGE

Table 2.1 Catalyst Type and Their Advantages and Disadvantages 14

Table 3.1 Analytical Methods and their Purpose 28

Table 3.2 GC Column Specifications and Operating Conditions 29

Table 4.1 Summary of BET Analysis for Both Catalyst Samples 40

viii

LIST OF FIGURES

PAGE

Figure 2.1 General Synthesis Steps for Bulk Catalyst and Support

Preparation

16

Figure 3.1 Schematic Diagram of Catalyst Testing Setup 24

Figure 3.2 Experimental Setup 27

Figure 4.1 XRD Pattern for Sample 1 of the Synthesized RuO2/ZrO2-

CeO2 Catalyst

32

Figure 4.2 XRD Pattern for Sample 2 of the Synthesized RuO2/ZrO2-

CeO2 Catalyst

33

Figure 4.3 XRD pattern of (a) RuO2/ZrO2, (b) RuO2/TiO2–CeO2, (c)

RuO2/ZrO2–CeO2, (d) RuO2/CeO2 and (e) RuO2/TiO2

34

Figure 4.4 Comparison of XRD Pattern of Sample 1 (Red), Sample 2

(Blue) and Catalyst from Wang et al. (Black)

35

Figure 4.5 Adsorption Isotherm for Catalyst Sample 1 37

Figure 4.6 BET Plot of Catalyst Sample 1 38

Figure 4.7 Adsorption Isotherm for Catalyst Sample 2 39

Figure 4.8 BET Plot of Catalyst Sample 2 40

Figure 4.7 Conversion Versus Time for Reaction Run at 60°C 42

Figure 4.8 Conversion Versus Time for Reaction Run at 70°C 43

Figure 4.9 Conversion Versus Time for Reaction Run at 80°C 44

Figure 4.10 Comparison of the Conversion of Acetic Acid at Different

Temperatures

45

Figure 4.11 Conversion Versus Time for Reaction Run at Air Flow

Rate of 0.1L/min

47

Figure 4.12 Conversion Versus Time for Reaction Run at Air Flow

Rate of 0.2L/min

48

Figure 4.13 Conversion Versus Time for Reaction Run at Air Flow

Rate of 0.3L/min

49

Figure 4.14 Comparison of the Conversion of Acetic Acid at Different

Air Flow Rates

50

ix

LIST OF ABBREVIATIONS

BET Brunauer, Emmett and Teller

CWAO Catalytic Wet Air Oxidation

GC Gas Chromatography

TOC Total Organic Carbon

WAO Wet Air Oxidation

XRD X–ray Diffraction

x

PEMANGKIN PENGOKSIDAAN UDARA BASAH AIR SISA

MENGANDUNGI ASID ASETIK

ABSTRAK

Air sisa mengandungi asid asetik telah lama dihasilkan oleh industri kimia.

Pelupusan sisa ini yang tidak betul telah menjadi masalah yang besar kerana sisa

tersebut mencemarkan alam sekitar dan memusnahkan ekosistem akua. Air sisa ini

harus dirawat sebelum dilepaskan kea lam sekitar. Antara kaedah rawatan termasuk

penggunaan membran dan pengoksidaan udara basah. Kaedah ini mempunyai

beberapa batasan. Proses membran terhad oleh kestabilan pelarut dan kestabilan

terma dan pengoksidaan udara basah hanya berkesan pada keadaan operasi yang

tinggi. Dalam kajian ini, pemangkin pengoksidaan udara basah adalah kaedah yang

dicadangkan untuk rawatan. Pemangkin telah disintesis berdasarkan kepada kaedah

kebasahan. Eksperimen telah dijalankan dengan mengoksidakan air kumbahan

simulasi mengandungi asid asetik dalam reaktor berkelompok. Kajian tindak balas

diulangi dengan manipulasi dua operasi parameter yang berbeza. Sampel telah

dianalisis menggunakan analisis Jumlah Karbon Organik dan Kromatografi Gas.

Pemangkin dicirikan oleh analisis Pembelauan Sinar X dan analisis Physisorption

(BET Method). Pemangkin pengoksidaan udara basah menghasilkan penukaran

tertinggi pada suhu 80°C. Pemangkin yang mempunyai kawasan permukaan tinggi

telah disahkan oleh analisis Pembelauan Sinar X dan Physisorption. Penukaran

menggunakan pemangkin pengoksidaan udara basah adalah lebih tinggi daripada

proses lain seperti yang diramalkan. Kehadiran pemangkin mengurangkan keterukan

keadaan operasi dan juga meningkat kadar penukaran.

xi

CATALYTIC WET AIR OXIDATION OF WASTEWATER CONTAINING

ACETIC ACID

ABSTRACT

Wastewater containing acetic acid has long been produced by the chemical industry.

Improper disposal of the wastewater has become a major problem as it pollutes the

environment and destroys aquatic ecosystem. The wastewater has to be treated

before it can be released into the environment. Some of the treatment methods

include membrane processes and wet air oxidation (WAO). These treatment methods

have a few limitations. Membrane processes are limited by their solvent and thermal

stability and WAO is only effective at severe operating conditions. In this research,

catalytic wet air oxidation (CWAO) is the proposed method for treatment. The

catalyst was synthesized according to the wetness method. The experiment was

conducted by oxidizing simulated wastewater containing acetic acid in a batch

reactor. The reaction study was repeated with the manipulation of two different

operating parameters. The sample was analysed using Total Organic Content (TOC)

analysis and Gas Chromatography (GC). The catalyst was characterized by X-ray

diffraction (XRD) and Physisorption analysis (BET Method). CWAO yielded the

highest conversion at the temperature of 80°C. The catalyst with high surface area

was confirmed by XRD and BET. The conversion using CWAO was higher than

other processes as predicted. The presence of the catalyst reduced the severity of the

operating conditions and also increased the conversion rate.

1

CHAPTER 1

INTRODUCTION

1.1 Background of the Proposed Study

Industrial processes normally produce wastewater and this industrial

wastewater is dangerous to be released into the environment. Some wastewater has

higher concentration of certain chemicals while some are more dilute. Organic

chemicals such as acetic acid are normally found in dilute industrial wastewater.

There are many industries that produce wastewater containing acetic acid. Among

these industries are the pharmaceutical industry, food and beverage industry (Kumar

and Babu, 2008) and polymer manufacturing industry (Shin et al., 2009). Acetic acid

is a weak acid and dilute wastewater containing acetic acid is harmful to the

environment as it can contaminate and destroy the aquatic ecosystem. Therefore,

industrial wastewater must be treated before being released into the environment. By

removing the acetic acid from the wastewater, we can, on one hand make full use of

our limited resources, and on the other, protect our environment (Yu et al., 2000).

2

Many methods have been utilized over the years to recover or remove acetic

acid from the wastewater. Simple separation techniques such as liquid–liquid

extraction, adsorption and distillation have proven to be ineffective (Kumar and

Babu, 2008). More advanced methods such as membrane and oxidation methods are

more widely used. One of the methods used to remove acetic acid is through wet air

oxidation (WAO). WAO is an oxidation process that oxidizes dissolvable or

suspended organic compounds as well as reducible inorganic compounds with

oxygen or air under the circumstances of high temperature and high pressure in

liquid phase. Catalytic wet air oxidation (CWAO) was a new technology developed

on the basis of WAO in the 1970s (Zhu et al., 2002). The usage of catalyst can

reduce the limitations of WAO by reducing the operating temperature, pressure and

also reduce the reaction time.

Previous methods of treatment such as WAO proved ineffective as WAO has

limited application due to the conditions of the process; high temperature, pressure

and long reaction time (Mikulová et al., 2007). Membrane processes has limited

solvent stability (Wee et al., 2008), thermal stability and are prone to fouling (Kumar

and Babu, 2008). This indirectly increases the cost of the process. Hence, CWAO is

a promising method and catalysts with high activity are needed to ensure higher

conversion and a more effective way to remove acetic acid.

3

1.2 Research Objectives

The objectives of the present study are:

a) To synthesize and characterize the ruthenium oxide on zirconium oxide and

cerium oxide support (RuO2/ZrO2-CeO2) catalyst.

b) To examine the activity of the synthesized catalyst in the catalytic wet air

oxidation of wastewater containing acetic acid.

1.3 Scope of the Study

The scopes of this study are the synthesis of RuO2/ZrO2-CeO2 catalyst using

the wetness method and determination of the activity of the synthesized catalyst in

oxidizing the wastewater containing acetic acid. The important parameters include

concentration of acetic acid in the wastewater, temperature and air flow rate. The

analysis methods used to determine the acetic acid concentration are Total Organic

Carbon (TOC) analysis and Gas Chromatography (GC). For the catalyst

characterization, X–ray diffraction (XRD) and Physisorption analysis (BET Method)

are conducted.

4

1.4 Significance of the Study

The significance of the proposed study is to synthesize the catalyst with high

activity in oxidizing the wastewater containing acetic acid. The synthesized catalyst

can be used in larger scale operations of treatment of wastewater containing acetic

acid. Besides that, the treated wastewater can be safely released into the environment

and prevent contamination and destruction of the aquatic ecosystem.

5

CHAPTER 2

LITERATURE REVIEW

This review discusses about industrial wastewater containing acetic acid,

treatment methods and their limitations, catalytic wet air oxidation (CWAO) method

and the type of catalyst used in CWAO.

2.1 Industrial Wastewater Containing Acetic Acid

A large number of chemical industries produce huge amounts of wastewater

containing various amounts of toxic and hazardous organic compounds. A typical

organic compound that is present in wastewater is carboxylic acid such as acrylic

acid and acetic acid. Detailed analysis shows that acetic acid is the most commonly

found organic acid with significant concentration. Acetic acid is not harmful to

6

humans if it is dilute, however, it is dangerous to the environment as it can

contaminate and destroy the aquatic ecosystem.

There are many industries that produce wastewater containing acetic acid.

Among these industries are the pharmaceutical industry, polymer manufacturing

industry, food and beverage industry. In a research done by Kumar and Babu (2008),

stated that acetic acid is most widely used in the field of food and beverages as an

acidulant. They also said acetic acid is used in the synthesis of acetyl cellulose and

plastics and also in the food industry, as well as in the printing and dyeing industries.

Slaughter house waste and animal by-products contains ammonia and organic

residue which include acetic acid. Animal by-products and slaughter house waste are

produced daily and must be dealt with to prevent pollution. The acetic acid in in

animal by-products can be treated by means of CWAO, as researched by Frontanier

et al. in 2010. Another source of acetic acid waste comes from the silicon wafer

manufacturing. The acetic acid is in the wafer etching process. Due to a rapid growth

of those industries in Korea, the amount of waste acids generated during etching and

cleaning processes is increasing rapidly (Shin et al., 2009).

2.2 Treatment Methods

Before wastewater can be released into the environment, it has to be treated

until it meets a standard set by the Department of Environment. Many methods have

been utilized over the years to recover or remove acetic acid from the wastewater.

7

Simple separation techniques such as adsorption and distillation have proven to be

ineffective. More advanced methods such as membrane and oxidation methods are

more widely used.

2.2.1 Adsorption and Distillation

Adsorption and distillation are well known separation methods. These

methods are more focused on removing the acetic acid from the water. Adsorption is

another good method to remove acetic acid but the cost associated with regeneration

of commercial adsorbents makes adsorption operation very expensive (Kumar and

Babu, 2008). Distillation has a disadvantage as it is only effective when the

concentration of acetic acid in the wastewater is high (Helsel, 1977).

2.2.2 Membrane Processes

Membrane process is an effective method to recover or remove organic

contaminants from wastewater. Basically, membrane process is a form of filtration

where wastewater goes through a membrane and the contaminants remain and

cleaner water is produced. Ultra filtration, reverse osmosis and pervaporation are

some the of membrane processes. The choice of the membrane is a key consideration

that defines the type of application (Jullok et al., 2011). Normally, the smallest

weight fraction of component in the mixture is to be transported across the

8

membrane: hydrophilic polymeric membranes are used for the dehydration of

organic liquids and hydrophobic polymeric membranes for removal of organics from

water streams (Kujawski, 2000). However, polymeric membranes have limitations.

Polymeric membrane has limited solvent stability (Wee et al., 2008) and thermal

stability. Besides that, another problem with membrane process is the membrane

fouling which requires frequent cleaning (Kumar and Babu, 2008).

2.2.3 Wet Air Oxidation

Wet air oxidation (WAO) is defined as the liquid phase oxidation of organic

compounds at temperatures (125–320°C) and pressures (0.5–20MPa) below the

critical point of water using a gaseous source of oxygen (Mishra et al., 1995). WAO

process is also defined as a thermochemical process where several active oxygen

species, including hydroxyl radicals, are formed at elevated temperatures (i.e. 200–

300°C) and pressures (i.e. 2–20MPa) (Katsoni et al., 2008).

This method works on the principles where low molecular weight pollutant

molecules such as acetic acid to carbon dioxide and water if the conditions are severe

enough. As a general rule, the oxidation rate increases with an increase in molecular

weight of the organic acid (Klinghoffer et al., 1998). Although this method is a well-

established technique, it has certain limitations. Severe operating conditions, low

oxidation rate of low molecular weight compounds and increased equipment and

operating costs (Verenich et al., 2000) are some of the limitations.

9

Several researches using WAO was conducted and investigated. WAO of

long-chain carboxylic acids such as caprylic and oleic acids was determine to be

effective where conversions of 90% and greater were achieved in 10 minutes

(Sánchez-Oneto et al., 2004). As stated previously, to achieve high conversion or

oxidation, operating conditions must be severe (Duprez et al., 2003 and Yang et al.,

2010).

2.2.4 Catalytic Wet Air Oxidation

Catalytic wet air oxidation (CWAO) is an improvement over the WAO

method with the introduction of a catalyst in the reaction. Introduction of a catalyst

into the reaction not only reduces the severity of the operating conditions such as

temperature (Wang et al., 2008), but also increases the oxidation rate. Homogeneous

and heterogeneous can be used in the reaction, but heterogeneous catalyst are

preferred because no catalyst recovery step is required (Klinghoffer et al., 1998).

This method overcomes the limitations of membrane and WAO techniques.

Many researches compared the efficiency of WAO and CWAO and how the

introduction of catalysts improves the oxidation process. Duprez et al. (2003)

concluded that CWAO processes, with homogeneous or heterogeneous catalysts,

require milder reaction conditions. Yang et al. (2010) investigated the efficiency of

WAO and CWAO in oxidizing of complex and high-loaded industrial wastewater

and concluded that CWAO is highly efficient for wastewater treatment.

10

2.3 Catalyst for Wet Air Oxidation

The usage of catalyst in reactions has long been established in the chemical

industry. Catalyst can either promote or inhibit a certain reaction. Catalysts that

promotes a reaction is called a promoter while catalysts that inhibits a reaction is

called an inhibitor. There are two types of catalyst, homogeneous catalyst and

heterogeneous catalyst. Homogeneous catalysts are catalysts that are in the same

phase as the reactants while heterogeneous catalysts are of different phase than the

reactants (Fogler, 2006). In catalytic wet air oxidation, the type of catalyst used is

heterogeneous catalyst. Heterogeneous catalysts are normally solids and are made

from metals. Suitable metals that used in the catalyst ranges from metal oxides (Font

et al., 1999 and Hung, 2009), transition metals (Gomes et al., 2005) and noble metals

(Barbier Jr. et al., 2010 and Wang et al., 2008).

2.3.1 Metal Oxides and Transition Metals

Metal oxides and transition metals have been used as catalyst in catalytic wet

air oxidation. The heterogeneous catalyst that have been used are Cu, Pd, CoO/ZnO,

Cu:Mn:La oxides on spinal supports (ZnO, Al2O3), copper chromite, iron oxide,

Co:Bi complex oxides and Mn/Ce (Klinghoffer et. al., 1998, Gomes et al., 2005,

Hung, 2009, Arena et al., 2012). Despite having established the effectiveness of

these metals as catalyst, they have also been proven to have some flaws. In a

research done by Mikulová et al. (2007), partial leaching of metal ions has been

11

observed during the reaction, and a recovery step is necessary. This additional

recovery step will increase the cost of wastewater treatment and this is undesirable.

2.3.2 Noble Metals

Noble metals are a class of metals that are highly resistant to corrosion and

oxidation. Supported noble metals (including Pt, Pd, Ru, and Rh) have been

proposed for the CWAO (Mikulová et al., 2007, Barbier Jr. et al., 2010, Wang et al.,

2008). Activity wise, Imamura et al. (1988) studied the catalytic effect of noble

metals on the wet oxidation of phenols and other model pollutant compounds, and

found that ruthenium, platinum and rhodium were more active than homogeneous

copper catalyst.

2.3.3 Potential Applications of RuO2/ZrO2-CeO2 Catalyst

This section reviews the potential application of the RuO2/ZrO2-CeO2

catalyst in other reaction systems. As this catalyst is used for CWAO of acetic acid

wastewater, this review will give a better insight the potential of this catalyst for the

CWAO of other types of wastewater.

12

2.3.3.1 CWAO of Phenolic Compounds

For the treatment of wastewater containing phenolic compounds such as

phenol, noble metal catalyst has been proven to work effectively to remove phenol.

Research done by Pintar et al. in 2008 shows that Ru/TiO2 catalyst not only enables

complete removal of phenol, but also removed total organic carbon (TOC) without

the formation of carbonaceous deposits. Barbier et al. (2005) also demonstrated the

activity order of CeO2 supported noble metals for the CWAO of phenol as follows:

Ru/CeO2> Pd/CeO2> Pt/CeO2

The introduction of ZrO2 into the CeO2 increases the mechanical strength,

specific surface area and adsorption capacity of the catalyst. When used in CWAO of

phenol, phenol and TOC removal stabilized approximately 100% and 96%

respectively (Wang et al., 2008). This shows that the RuO2/ZrO2-CeO2 catalyst can

be applied to CWAO of phenolic compounds.

2.3.3.2 CWAO of N-Containing Compounds

Nitrogen-containing compounds are normally present in organic waste and

are highly toxic as they can cause acidification of the ecosystem. Some nitrogen-

containing compounds such as ammonia and aniline can be treated using CWAO.

Aniline is a representative compound of N-containing aromatic compounds and is

mainly used as a chemical intermediate in the production of polymers, pesticides,

pharmaceuticals, and dyes (Ersöz and Atalay, 2010).

13

Many researchers have utilized ruthenium catalyst in the CWAO of aniline.

Barbier et al. (2005) used a Ru/CeO2 catalyst while Reddy and Mahajani (2005) used

a Ru/SiO2 catalyst in the CWAO of aniline. As ruthenium is applicable as a catalyst

for the conversion of aniline, RuO2/ZrO2-CeO2 has great potential in the reaction of

aniline as it may improve on the current reaction rate and conditions.

Ammonia is widely used as a chemical in the manufacture of ammonium

nitrate, metallurgy, petroleum refineries, etc. It is known as a key intermediate in the

oxidation of N-containing compounds and is not amenable to direct biological

treatment due to its toxicity (Ihm and Kim, 2011). Barbier et al. (2002) showed that

CWAO is a very effective way to removal of ammonia where ammonia is converted

in to elemental nitrogen.

Other research also used ruthenium as a catalyst in the oxidation of ammonia.

Ru/TiO2 was used to remove ammonia (Lee et al., 2005) and it was concluded in that

research that ruthenium catalyst was responsible for the oxidation of ammonia and

does not affect the selectivity of the formation of nitrogen. RuO2/ZrO2-CeO2 catalyst

can be researched to see whether it can oxidize ammonia.

In this research, the researcher is using a ruthenium oxide on zirconium oxide

and cerium oxide support (RuO2/ZrO2-CeO2) catalyst. The selection of ruthenium as

the catalyst is based on its resistance towards sintering by carbonaceous species

during CWAO. Therefore, compared to the platinum catalysts, the equivalent

ruthenium materials demonstrate higher resistance against poisoning by

carbonaceous species during the CWAO experiments (Gaálová et al., 2010).

Ruthenium catalyst has more significant activity compared to platinum catalyst and

this is proven in a research done by Perkas et al. (2005).

14

Table 2.1 Catalyst Type and Their Advantages and Disadvantages.

Catalyst Type Advantage Disadvantage

Metal Oxides and

Transition Metals

Cost of catalyst is cheaper

compared to noble metal

catalyst

Partial leaching of metal ion

and recovery step is needed

(Mikulová et al., 2007)

Noble Metals High activity (Imamura et al.,

1988) , resistant to poisoning

by carbonaceous species

(Gaálová et al., 2010)

Noble metal catalyst is more

expensive than metal oxides

and transition metal catalysts

2.4 Catalyst Synthesis Methods

There are many methods to synthesize catalyst and many researchers utilize

different methods to synthesize their own type of catalyst, specific to their research.

The choice of synthesis methods depends on the final catalyst desired, especially its

physical and chemical composition. Generally, the methods may contain the same

procedure, but the overall method is unique by itself. In a research conducted by

Perego and Villa in 1997, they discussed on the catalyst preparation method. Their

paper included on the different methods used to prepare catalysts which are bulk

catalyst and support preparation and impregnation method, which will be reviewed

in the following subtopics.

15

2.4.1 Bulk Catalyst and Support Preparation

Bulk catalysts are actually the active substance or the active precursors of the

catalyst. Examples of bulk catalyst include silica-alumina for hydrocarbon cracking,

Zn-Cr oxide for the conversion of CO-H2 mixtures to methanol and iron-molybdate

for methanol oxidation (Perego and Villa, 1997). Bulk catalysts are further divided

into two different types of preparation which are precipitation and sol-gel method.

Both these methods have similar steps, but the precipitate that forms separates these

two methods.

2.4.1.1 Precipitation Method

Bulk catalyst synthesized using the precipitation method is a simple synthesis

method where the active substance is precipitated from a liquid solution and is the

base for the catalyst itself. Generally, the precipitate contains the active substance

and will slowly become the final, desired catalyst as it progresses.